It is known, particularly in oil exploration, to produce seismic images from a series of geophysical measurements conducted from the surface of a subsoil region. In the seismic technique, these measurements involve emitting a wave into the subsoil and measuring a signal containing reflections of the wave on the geological structures encountered. These structures are typically the surfaces separating different materials or faults.
Seismic images are representations of the subsoil in two or three dimensions, with the vertical dimension corresponding either to the propagation times of the seismic waves, or to the depths. They are obtained by techniques known by the term “migration” which use a model of estimated velocity providing a map of the seismic wave propagation speed in the rocks constituting the area being explored. This velocity model is used to estimate the positions of the reflectors in the subsoil based on seismic recordings. The seismic images produced in this way have some distortions of course, as do the underlying velocity models, because these are only estimates derived from a necessarily limited number of measurements.
In the case of marine subsurface exploration, seismic wave detectors are generally placed at the bottom of the sea on the subsoil to be explored. Seismic waves are emitted from the ocean surface. These waves propagate in the water and enter the subsoil. The detectors placed on the seabed on the surface of the subsoil will detect the arrival of the direct seismic wave as well as the waves reflected by the subsoil.
In order to monitor the evolution of a oil reservoir in the subsurface, it is possible to obtain a first seismic image of the subsoil at a given moment then obtain a second seismic image of the same subsoil after a certain amount of time.
In particular, to track changes in hydrocarbon content of a reservoir in production, it can be useful to monitor the evolution of the seismic image of the subsoil over time.
In order to be able to compare two seismic images of the same subsoil region, it is important to know how to position each detector on the surface of the subsoil as accurately as possible.
The detectors are generally positioned at the bottom of the sea at a depth of several hundred meters using a Remotely Operated Vehicle (ROV) controlled from the surface. However, the operating constraints on deploying such vehicles combined with the accuracy of their onboard acoustic positioning systems, which require long stabilization and calibration times, commonly lead to inaccurate positioning of the receiver relative to the planned position.
Generally, the position of the detector is only known to a precision of about 10 meters.
In a context where sets of measurements are collected at different times, this implies an uncertainty of 20 m in the position of the detector, which considerably reduces the repeatability of the measurements.
It is possible to determine the position of a detector by triangulation. Three seismic waves are emitted from three points on the surface and the distance between the detector and the coordinates of each emission point is calculated based on the travel time of the seismic wave.
The accuracy of such a method is based on knowing both the bathymetry and the seismic wave propagation speed in water. This propagation speed can vary substantially, particularly as the water temperature and salinity vary. Also, the bathymetry is generally measured using acoustic means which are themselves dependent on the speed in water and other parameters. The accuracy of the triangulation method will therefore vary substantially from one set of measurements to another.
A need therefore exists for a means of more accurately positioning detectors placed at the bottom of the sea, based on neither knowledge of the wave propagation speed in water nor on the bathymetry. It is sufficient for this method to determine the position in a plane, because it is known that the detectors are placed on the surface of the seabed.